This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-146500 filed on May 21, 2002.
The present invention relates to a diaphragm-type semiconductor pressure sensor, which includes a semiconductor substrate having: active surface and back surface of (110) crystallographic face orientation; and a diaphragm that has been formed in the active surface by forming a recess in the back surface, and relates to a semiconductor wafer used for manufacturing the diaphragm-type semiconductor pressure sensor.
The diaphragm-type semiconductor pressure sensor includes a semiconductor substrate that has an active surface of (110) crystallographic face orientation and a back surface, which is opposite to the active surface, of (110) crystallographic face orientation. Hereafter, this type of semiconductor substrate will be referred as a (110) semiconductor substrate.
As shown in FIG. 12, a proposed diaphragm-type semiconductor pressure sensor includes a rectangular (110) semiconductor substrate 10 having four sides 10a. The (110) semiconductor substrate 10 includes a diaphragm 14 used for detecting a pressure. The diaphragm 14 is located at a bottom of a recess 13, or in the active surface of the (110) semiconductor substrate 10. The recess 13 has been formed by an isotropically etching a portion of a silicon substrate, from which the (110) semiconductor substrate has been formed, from the back surface thereof.
The diaphragm 14 includes gauge resistors Rc1, Rc2, Rs1, Rs2, which are piezoresistive elements. As shown in FIG. 12, the gauge resistors Rc1, Rc2, Rs1, Rs2 are made up of two center gauge resistors Rc1, Rc2, which are located at the central area of the diaphragm 14, and two side gauge resistors Rs1, Rs2, which are located at the periphery of the diaphragm 14. The four gauge resistors Rc1, Rc2, Rs1, Rs2 make up a bridge circuit used for detecting the pressure. When the diaphragm 14 is strained by a pressure to be detected, the resistances of the gauge resistors Rc1, Rc2, Rs1, Rs2 vary in response to the strain of the diaphragm 14, and the pressure is detected on the basis of the variation in the resistances.
In the manufacturing process of the proposed diaphragm-type semiconductor pressure sensor, a plurality of rectangular regions, which become sensor chips, are formed in a silicon wafer, which has an active surface of (110) crystallographic face orientation, a back surface, which is opposite to the active surface, of (110) crystallographic face orientation, and an orientation flat having a crystallographic face of (100) orientation. The regions are defined by forming scribe lines substantially parallel to the orientation flat and scribe lines substantially orthogonal to the orientation flat. Then, gauge resistors Rc1, Rc2, Rs1, Rs2 are formed using semiconductor process techniques such as ion implantation and diffusion in the area of each of the regions where a diaphragm 14 is to be formed. Next, a portion of the silicon wafer is anisotropically etched from the back surface in each of the regions to form a recess 13 and simultaneously the diaphragm 14 in the active surface of the silicon wafer. With the above steps, a (110) semiconductor wafer is formed. Finally, the (110) semiconductor wafer is diced into a plurality of semiconductor pressure sensors shown in FIG. 12.
In the semiconductor pressure sensor of FIG. 12, in which a (110) semiconductor substrate is used, the strain of the diaphragm 14 is used for detecting the pressure applied to the diaphragm 14, as described above. Two crystallographic axes of  less than 110 greater than  and  less than 100 greater than  orientations exist on a crystallographic plane of (100) orientation. However, the piezoresistive coefficient of silicon along a crystallographic axis of  less than 110 greater than  orientation is much greater, for example, about fifty times greater, than that along a crystallographic axis of  less than 100 greater than  orientation. That is, the sensitivity in detecting the strain generated along a crystallographic axis of  less than 110 greater than  orientation is much greater than that along a crystallographic axis of  less than 100 greater than orientation. Therefore, the gauge resistors Rc1, Rc2, Rs1, Rs2 have been formed such that the gauge resistors Rc1, Rc2, Rs1, Rs2 substantially extend along a crystallographic axis of  less than 110 greater than  orientation in the semiconductor pressure sensor of FIG. 12 in order to increase the sensitivity.
A crystallographic plane of (100) orientation includes only one crystallographic axis of  less than 110 greater than  orientation, so the arrangement of the gauge resistors Rc1, Rc2, Rs1, Rs2 shown in FIG. 12 is substantially the best to gain the highest sensitivity in pressure detection. The pressure sensor of FIG. 12 has been bonded to a sealing substrate such as a glass stand, which is not shown in the figure, at the back surface of the (110) semiconductor substrate 10 using anodic bonding and so on such that the recess 13 is hermetically sealed by the sealing substrate to form a pressure reference room.
Lately, there have been demands for shrinking the semiconductor pressure sensor of FIG. 12 for the purpose of cost reduction and soon. To shrink the semiconductor pressure sensor of FIG. 12, the (110) semiconductor substrate 10 needs to be shrunk.
However, if the (110) semiconductor substrate 10 was shrunk with simply shrinking the diaphragm 14 without changing layout, the sensitivity in pressure detection would worsen. Even if the (110) semiconductor substrate 10 was shrunk without shrinking the diaphragm 14 or changing layout, the minimum width L of the contact area between the back surface of the (110) semiconductor substrate 10 and the sealing substrate would become narrower. That is, the frame-like portion of the (110) semiconductor substrate 10, which surrounds the diaphragm 14, needs to be narrowed.
The hermeticity of the pressure reference room is expressed using the molecular leak rate equation (1) in vacuum engineering,
Q=(2xcfx80V/3)xc3x97r3xc3x97(P1xe2x88x92P2)/Lxe2x80x83xe2x80x83(1)
where Q is the leak rate of the pressure reference room, r is the radius of a leak passage LP at the boundary between the back surface of the (110) semiconductor substrate 10 and the sealing substrate, L is the length of the leak passage LP, or the above-mentioned minimum width of the back surface, V is the average velocity of gas molecules, P1 is the pressure outside the pressure reference room, and P2 is the pressure in the pressure reference room. As understood from the equation (1), the leak rate Q is inversely proportionate to the length L of the leak passage LP. Therefore, if the (110) semiconductor substrate 10 was shrunk without shrinking the diaphragm 14 or changing layout, it would become difficult to assure the hermeticity of the pressure reference room. As a result, the reliability of the pressure sensor of FIG. 12 would worsen.
The present invention has been made in view of the above aspects. A first object of the present invention is to shrink a diaphragm-type semiconductor pressure sensor without shrinking the diaphragm thereof or shortening the minimum width of the back surface thereof in order to make the most of the dimensions of the sensor. A second object of the present invention is to provide a semiconductor wafer that can be used to shrink a diaphragm-type semiconductor pressure sensor in order to make the most of the dimensions of the sensor.
To achieve the first object, a diaphragm-type semiconductor pressure sensor according to the present invention includes a substantially rectangular (110) semiconductor substrate, which has four sides, an active surface of (110) crystallographic face orientation, and a back surface, which is opposite to the active surface, of (110) crystallographic face orientation. Each of the surfaces is surrounded by the four sides. Each of the four sides is at an angle of substantially 45 degrees with a crystallographic axis of  less than 110 greater than  orientation that is substantially parallel to the active surface. The substrate includes a diaphragm in the active surface. The diaphragm has been formed by forming a recess in the back surface. The diaphragm includes a gauge resistor. A pressure is detected on the basis of the variation in the resistance of the gauge resistor.
To achieve the second object, a semiconductor wafer according to the present invention that is used for manufacturing a diaphragm-type semiconductor pressure sensor includes a (110) semiconductor layer. The (110) semiconductor layer includes an active surface of (110) crystallographic face orientation, and a back surface, which is opposite to the active surface, of (110) crystallographic face orientation, and an orientation flat. A plurality of scribe lines are located on the active surface. The orientation flat is at an angle of substantially 45 degrees with a crystallographic face of (100) orientation that is substantially orthogonal to the active surface of the (110) semiconductor layer. Each of the scribe lines is substantially parallel or orthogonal to the orientation flat. The (110) semiconductor layer also includes substantially rectangular regions, which are defined by the scribe lines. Each of the regions includes a diaphragm in the active surface. The diaphragm has been formed by forming a recess in the back surface. The diaphragm includes a gauge resistor. A pressure is detected on the basis of the variation in the resistance of the gauge resistor in a diaphragm-type semiconductor pressure sensor manufactured from the semiconductor wafer.